Background <p>Malignant pleural effusion due to metastatic squamous cell carcinoma is uncommon. Keratinizing squamous cell carcinoma is morphologically identifiable, whereas nonkeratinizing squamous cell carcinoma poses diagnostic challenges and is prone to underdiagnosis or misdiagnosis.</p> Case presentation <p>This study reports the case of a 66-year-old East Asian man with a history of esophageal squamous cell carcinoma who developed pleural metastasis 3&#xa0;years after chemotherapy, presenting with persistent, dull right-sided chest pain of unknown origin. Chest computed tomography revealed a large left pleural effusion with partial atelectasis. Approximately 600&#xa0;ml of bloody fluid was drained via thoracentesis and catheter drainage. Cytological examination showed tumor cells exhibiting ring-like and pseudoglandular structure. The final cytological diagnosis was metastatic squamous cell carcinoma within the pleural effusion. The patient opted for symptomatic and conservative management and died 9&#xa0;months after malignant pleural effusion diagnosis during follow-up.</p> Conclusion <p>When metastatic squamous cell carcinoma in pleural effusion is poorly differentiated and exhibits atypical changes such as pseudoglandular and signet ring-like features, vigilance is required during serous fluid cytopathological diagnosis. An accurate diagnosis provides the basis for subsequent clinical treatment.</p>